1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens; the zoom lens may be suitable for use as a photographic optical system in, for example, a single-lens reflex camera, a digital still camera, a digital video camera, a TV camera, a monitoring camera, or the like.
2. Description of the Related Art
There is an increasing demand for an image pickup apparatus, such as a single-lens reflex camera, a video camera, or the like, capable of performing autofocusing (AF) at high speed with high accuracy. Conventionally, phase difference autofocusing has been widely used as an autofocusing method for capturing a still image. On the other hand, in recent single-lens reflex cameras, functions of capturing a moving image and autofocusing during capturing a moving image have been demanded. As an autofocusing method for use in capturing a moving image, contrast type autofocusing has been widely used. In the contrast type autofocusing, a focusing state of a photographic optical system is evaluated by detecting a high frequency component of a captured image signal.
In an image pickup apparatus using the contrast type autofocusing method, the direction shifted from an in-focus state is detected by vibrating a focus lens unit in an optical axis direction at a high speed (hereinafter, referred to as “wobbling”). Next, after the wobbling, an optimal position of the focus lens unit, that is, the in-focus state, is calculated by detecting a signal component of a specific frequency band of an image area, from an output signal of an image sensor. After that, the focus lens unit is moved to the optimal position, so that the focusing is completed.
In the case of capturing a moving image, in order to reduce the focusing time, the focus lens unit needs to be driven at a high speed. In addition, the focus lens unit needs to be driven as quietly as possible so that a driving sound of a motor cannot be recorded. Therefore, in order to minimize a load of the motor, a small and light-weight focus lens unit has been demanded. In an image pickup apparatus using a phase difference autofocusing method, the use of a small and light focus lens unit is also demanded. There has been known a zoom lens which performs focusing by partially adopting the small and light lens units among the entire lens units constituting the zoom lens.
U.S. Pat. No. 7,184,221 discusses, for example, a zoom lens in which a negative lens unit and a positive lens unit are disposed at an image side in this order and focusing is performed by the negative lens unit, which is small and light-weight. U.S. Patent Application Publication No. 2010/0091170 discusses a zoom lens including first to fifth lens units having positive, negative, negative, positive, and positive refractive powers, respectively, in order from the object side to the image side, in which focusing is performed by the third lens unit. U.S. Patent Application Publication No. 2010/0091171 discusses a five-unit zoom lens including first to fifth lens units having positive, negative, positive, positive, and positive refractive powers, respectively, in which focusing is performed by using the third lens unit. U.S. Patent Application Publication No. 2011/0116174 discusses a four-unit zoom lens including first to fourth lens units having positive, negative, positive, and positive refractive powers, respectively, in which focusing is performed by using the second lens unit.
In order to improve the autofocusing speed, it is useful to use a small and light lens unit composed of a small number of lenses as a focus lens unit. In addition, in order to reduce an amount of movement during focusing, it is useful to use a lens unit having a strong refractive power as a focus lens unit. In general, in the case where the focus lens unit is configured by using a small number of lenses, if the power (refractive power) of the focus lens unit is strengthened, the residual aberration of the focus lens unit increases, which is accordingly accompanied by variations in aberration due to focusing. Therefore, the power of the focus lens unit cannot be strengthened by an excessive amount.
On the other hand, if the power of the focus lens unit is weakened, the amount of movement during focusing is increased. As a result, the space where the focus lens unit is moved is increased, so that sufficient space where the zooming lens unit can be moved cannot be secured. Therefore, it is difficult to allow the entire zoom lens to be accommodated in a small size while maintaining high optical performance. In U.S. Patent Application Publication No. 2010/0091170, the third lens unit having a negative refractive power is configured as a focus lens unit, and in U.S. Patent Application Publication No. 2010/0091171, the third lens unit having a positive refractive power is configured as a focus lens unit.
In these zoom lenses, to allow the amount of movement of the focus lens unit to be decreased, the negative power of the second lens unit in U.S. Patent Application Publication No. 2010/0091170 or the positive power of the fourth lens unit in U.S. Patent Application Publication No. 2010/0091171 is weakened to a degree that the power of the focus lens unit is strengthened. Therefore, in U.S. Patent Application Publication No. 2010/0091170, in the case where a composite lens unit of the second lens unit and the third lens unit is configured as a lens unit having a negative refractive power, the principal point position is greatly shifted to the image side with respect to the second lens unit having a negative refractive power. In addition, in U.S. Patent Application Publication No. 2010/0091171, in the case where a composite lens unit of the third lens unit and the fourth lens unit is configured as a lens unit having a positive refractive power, the principal point position is greatly shifted to the object side with respect to the third lens unit having a positive refractive power.
As a result, it is difficult to appropriately set the power of each lens unit for correcting the variation of aberration during zooming, so that there is a tendency that it is difficult to implement high performance and to allow the entire zoom lens to have a small size.